Space exploration and ground-based observations have provided
outstanding evidence of the diversity and the complexity of the outer
solar system. This work presents our current understanding of the nature
and distribution of water and water-rich materials from the water snow
line to the Kuiper Belt. This synthesis is timely, since a thorough
exploration of at least one object in each region of the outer solar
system has now been achieved. Next steps, starting with the Juno mission
now in orbit around Jupiter, will be more focused on understanding the
processes at work than on describing the general characteristics of each
giant planet systems.

The origin of the orbital structure of the cold component of the Kuiper
belt is still a hot subject of investigation. Several features of the
solar system suggest that the giant planets underwent a phase of global
dynamical instability, but the actual dynamical evolution of the planets
during the instability is still debated. To explain the structure of the
cold Kuiper belt, Nesvorny (2015, AJ 150,68) argued for a "soft"
instability, during which Neptune never achieved a very eccentric orbit.
Here we investigate the possibility of a more violent instability, from
an initially more compact fully resonant configuration of 5 giant
planets. We show that the orbital structure of the cold Kuiper belt can
be reproduced quite well provided that the cold population formed in
situ, with an outer edge between 44-45 au and never had a large mass.

Images of the Kuiper Belt object (126719) 2002 CC249 obtained in
2016 and 2017 using the 6.5 m Magellan-Baade Telescope and the 4.3 m
Discovery Channel Telescope are presented. A light curve with a
periodicity of 11.87±0.01 hr and a peak-to-peak amplitude of
0.79±0.04 mag is reported. This high amplitude double-peaked light
curve can be due to a single elongated body, but it is best explained by
a contact binary system from its U-/V-shaped light curve. We present a
simple full-width-at-half-maximum test that can be used to determine if
an object is likely a contact binary or an elongated object based on its
light curve. Considering that 2002 CC249 is in hydrostatic
equilibrium, a system with a mass ratio qmin = 0.6, and a density
ρmin = 1 g cm−3, or less plausible a system with
qmax = 1, and ρmax = 5 g cm−3 can interpret the
light curve. Assuming a single Jacobi ellipsoid in hydrostatic
equilibrium and an equatorial view, we estimate ρ ≥ 0.34 g cm−3,
and a/b = 2.07. Finally, we report a new color study showing
that 2002 CC249 displays an ultra red surface characteristic of a
dynamically Cold Classical trans-Neptunian object.

We measured the mean plane of the Kuiper belt as a function of
semi-major axis. For the classical Kuiper belt as a whole (the
non-resonant objects in the semi-major axis range 42-48 au), we find a
mean plane of inclination
im = 1.8°+0.7°−0.4° and longitude of
ascending node Ωm = 77°+18°−14° (in
the J2000 ecliptic-equinox coordinate system), in accord with
theoretical expectations of the secular effects of the known planets.
With finer semi-major axis bins, we detect a statistically significant
warp in the mean plane near semi-major axes 40-42 au. Linear secular
theory predicts a warp near this location due to the ν18 nodal
secular resonance, however the measured mean plane for the 40.3-42 au
semi-major axis bin (just outside the ν18) is inclined
∼ 13° to the predicted plane, a nearly 3-σ
discrepancy. For the more distant Kuiper belt objects of semi-major axes
in the range 50-80 au, the expected mean plane is close to the
invariable plane of the solar system, but the measured mean plane
deviates greatly from this: it has inclination
im = 9.1°+6.6°−3.8° and longitude of
ascending node Ωm = 227°+18°−44°. We
estimate this deviation from the expected mean plane to be statistically
significant at the ∼ 97−99% confidence level. We discuss several
possible explanations for this deviation, including the possibility that
a relatively close-in (a <~100 au), unseen small planetary-mass
object in the outer solar system is responsible for the warping.

The observational census of trans-Neptunian objects with semi-major axes
greater than ∼ 250 AU exhibits unexpected orbital structure that is
most readily attributed to gravitational perturbations induced by a
yet-undetected, massive planet. Although the capacity of this planet to
(i) reproduce the observed clustering of distant orbits in physical
space, (ii) facilitate dynamical detachment of their perihelia from
Neptune, and (iii) excite a population of long-period centaurs to
extreme inclinations is well established through numerical experiments,
a coherent theoretical description of the dynamical mechanisms
responsible for these effects remains elusive. In this work, we
characterize the dynamical processes at play, from semi-analytic
grounds. We begin by considering a purely secular model of orbital
evolution induced by Planet Nine, and show that it is at odds with the
ensuing stability of distant objects. Instead, the long-term survival of
the clustered population of long-period KBOs is enabled by a web of
mean-motion resonances driven by Planet Nine. Then, by taking a
compact-form approach to perturbation theory, we show that it is the
secular dynamics embedded within these resonances that regulates the
orbital confinement and perihelion detachment of distant Kuiper belt
objects. Finally, we demonstrate that the onset of large-amplitude
oscillations of orbital inclinations is accomplished through capture of
low-inclination objects into a high-order secular resonance and identify
the specific harmonic that drives the evolution. In light of the
developed qualitative understanding of the governing dynamics, we offer
an updated interpretation of the current observational dataset within
the broader theoretical framework of the Planet Nine hypothesis.

The organization of the orbits of most minor bodies in the Solar system
seems to follow random patterns, the result of billions of years of
chaotic dynamical evolution. Much as heterogeneous orbital behaviour is
ubiquitous, dynamically coherent pairs and groups of objects are also
present everywhere. Although first studied among the populations of
asteroids and comets that inhabit or traverse the inner Solar system,
where they are very numerous, at least one asteroid family has been
confirmed to exist in the outer Solar system and two other candidates
have been proposed in the literature. Here, we perform a systematic
search for statistically significant pairs and groups of dynamically
correlated objects through those with semimajor axis greater than 25 au,
applying a novel technique that uses the angular separations of orbital
poles and perihelia together with the differences in time of perihelion
passage to single out pairs of relevant objects. Our analysis recovers
well-known, dynamically coherent pairs and groups of comets and
trans-Neptunian objects and uncovers a number of new ones, prime
candidates for further spectroscopic study.

Published in:
Monthly Notices of the Royal Astronomical Society, 474, 838 (2018 Feb)

The surprising discovery of a ring system around the Centaur 10199 Chariklo
in 2013 led to a reanalysis of archival stellar occultation
data for the Centaur 2060 Chiron by Ortiz et al. One possible
interpretation of that data is that a system of rings exists around
Chiron. In this work, we study the dynamical history of the proposed
Chiron ring system by integrating nearly 36,000 clones of the Centaur
backward in time for 100 Myr under the influence of the Sun and the four
giant planets. The severity of all close encounters between the clones
and planets while the clones are in the Centaur region is recorded,
along with the mean time between close encounters. We find that severe
and extreme close encounters are very rare, making it possible that the
Chiron ring system has remained intact since its injection into the
Centaur region, which we find likely occurred within the past 8.5 Myr.
Our simulations yield a backward dynamical half-life for Chiron of 0.7 Myr.
The dynamical classes of a sample of clones are found. It is found
that, on average, the Centaur lifetimes of resonance hopping clones are
twice those of random-walk clones because of resonance sticking in mean
motion resonances. In addition, we present MEGNO and chaotic lifetime
maps of the region bound by 13 au ≤ a ≤ 14 au and
e ≤ 0.5. We confirm that the current mean orbital parameters of
Chiron are located in a highly chaotic region of a-e phase space.

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is currently
the deepest wide-field survey in progress. The 8.2 m aperture of the
Subaru telescope is very powerful in detecting faint/small moving
objects, including near-Earth objects, asteroids, centaurs and
Tran-Neptunian objects (TNOs). However, the cadence and dithering
pattern of the HSC-SSP are not designed for detecting moving objects,
making it difficult to do so systematically. In this paper, we introduce
a new pipeline for detecting moving objects (specifically TNOs) in a
non-dedicated survey. The HSC-SSP catalogs are sliced into HEALPix
partitions. Then, the stationary detections and false positives are
removed with a machine learning algorithm to produce a list of moving
object candidates. An orbit linking algorithm and visual inspections are
executed to generate the final list of detected TNOs. The preliminary
results of a search for TNOs using this new pipeline on data from the
first HSC-SSP data release (Mar 2014 to Nov 2015) present
231 TNO/Centaurs candidates. The bright candidates with Hr < 7.7
and i > 5 show that the best fit slope of a single power law to
absolute magnitude distribution is 0.77. The g−r color distribution of
hot HSC-SSP TNOs indicates a bluer peak at g−r = 0.9 which is
consistent with the bluer peak of the bimodal color distribution in
literature.

To appear in:
Publications of the Astronomical Society of Japan, HSC special issue

Pipeline for the Detection of Serendipitous Stellar Occultations by Kuiper Belt Objects with the Colibri Fast-photometry Array

E. Pass1,2,3, S. Metchev1,3,4, P. Brown1,3, and S. Beauchemin5

1 Department of Physics and Astronomy, University of Western Ontario, London ON, Canada
2 Department of Physics and Astronomy, University of Waterloo, Waterloo ON, Canada
3 Centre for Planetary Science and Exploration, University of Western Ontario, London ON, Canada
4 Department of Physics and Astronomy, Stony Brook University, Stony Brook NY, USA
5 Department of Computer Science, University of Western Ontario, London ON, Canada

We report results from the preliminary trials of Colibri, a dedicated
fast-photometry array for the detection of small Kuiper belt objects
through serendipitous stellar occultations. Colibri's novel data
processing pipeline analyzed 4000 star hours with two overlapping-field
EMCCD cameras, detecting no Kuiper belt objects and one false positive
occultation event in a high ecliptic latitude field. No occultations
would be expected at these latitudes, allowing these results to provide
a control sample for the upcoming main Colibri campaign. The empirical
false positive rate found by the processing pipeline is consistent with
the 0.002% simulation-determined false positive rate. We also describe
Colibri's software design, kernel sets for modeling stellar
occultations, and method for retrieving occultation parameters from
noisy diffraction curves. Colibri's main campaign will begin in
mid-2018, operating at a 40 Hz sampling rate.

Published in:
Publications of the Astronomical Society of the Pacific, 130, 014502

The large trans-Neptunian objects (TNO) with radii larger than 400 km
are thought to be in hydrostatic equilibrium. Their shapes can provide
clues regarding their internal structures that would reveal information
on their formation and evolution. In this paper, we explore the
equilibrium figures of five TNOs, and we show that the difference
between the equilibrium figures of homogeneous and heterogeneous
interior models can reach several kilometers for fast rotating and low
density bodies. Such a difference could be measurable by ground-based
techniques. This demonstrates the importance of developing the shape up
to second and third order when modeling the shapes of large and rapid
rotators.

We examine the relevance of tidal heating for large Trans-Neptunian
Objects, with a focus on its potential to melt and maintain layers of
subsurface liquid water. Depending on their past orbital evolution,
tidal heating may be an important part of the heat budget for a number
of discovered and hypothetical TNO systems and may enable formation of,
and increased access to, subsurface liquid water. Tidal heating induced
by the process of despinning is found to be particularly able to compete
with heating due to radionuclide decay in a number of different
scenarios. In cases where radiogenic heating alone may establish
subsurface conditions for liquid water, we focus on the extent by which
tidal activity lifts the depth of such conditions closer to the surface.
While it is common for strong tidal heating and long lived tides to be
mutually exclusive, we find this is not always the case, and highlight
when these two traits occur together. We find cases where TNO systems
experience tidal heating that is a significant proportion of, or greater
than radiogenic heating for periods ranging from 100's of millions to a
billion years. For subsurface oceans that contain a small antifreeze
component, tidal heating due to very high initial spin states may enable
liquid water to be preserved right up to the present day. Of particular
interest is the Eris-Dysnomia system, which in those cases may exhibit
extant cryovolcanism.

We present the results of photometric observations of six Transneptunian objects
and three Centaurs, estimations of their rotational periods and corresponding
amplitudes. For six of them we present also lower limits of density values. All
observations were made using 3.6-m TNG telescope (La Palma, Spain). For four
objects - (148975) 2001 XA255, (281371) 2008 FC76, (315898) 2008 QD4, and 2008 CT190
- the estimation of short-term variability was made for the first time.
We confirm rotation period values for two objects: (55636) 2002 TX300 and
(202421) 2005 UQ513, and improve the precision of previously reported
rotational period values for other three - (120178) 2003 OP32, (145452) 2005 RN43,
(444030) 2004 NT33 - by using both our and literature data. We also discuss
here that small distant bodies, similarly to asteroids in the Main belt, tend
to have double-peaked rotational periods caused by the elongated shape rather
than surface albedo variations.

Published in:
Monthly Notices of the Royal Astronomical Society, 474, 2536 (2018 Feb)

Pluto's atmosphere is cold and hazy. Recent observations have shown it
to be much colder than predicted theoretically, suggesting an unknown
cooling mechanism. Atmospheric gas molecules, particularly water vapour,
have been proposed as a coolant; however, because Pluto's thermal
structure is expected to be in radiative-conductive equilibrium, the
required water vapour would need to be supersaturated by many orders of
magnitude under thermodynamic equilibrium conditions. Here we report
that atmospheric hazes, rather than gases, can explain Pluto's
temperature profile. We find that haze particles have substantially
larger solar heating and thermal cooling rates than gas molecules,
dominating the atmospheric radiative balance from the ground to an
altitude of 700 km, above which heat conduction maintains an isothermal
atmosphere. We conclude that Pluto's atmosphere is unique amongst Solar
System planetary atmospheres, as its radiative energy equilibrium is
controlled primarily by haze particles instead of gas molecules. We
predict that Pluto is therefore several orders of magnitude brighter at
mid-infrared wavelengths than previously thought - a brightness that
could be detected by future telescopes.

A search for temporal changes on Pluto and Charon was motivated by (1)
the discovery of young surfaces in the Pluto system that imply ongoing
or recent geologic activity, (2) the detection of active plumes on
Triton during the Voyager 2 flyby, and (3) the abundant and detailed
information that observing geologic processes in action provides about
the processes. A thorough search for temporal changes using New
Horizons images was completed. Images that covered the same region were
blinked and manually inspected for any differences in appearance. The
search included full-disk images such that all illuminated regions of
both bodies were investigated and higher resolution images such that
parts of the encounter hemispheres were investigated at finer spatial
scales. Changes of appearance between different images were observed
but in all cases were attributed to variability of the imaging
parameters (especially geometry) or artifacts. No differences of
appearance that are strongly indicative of a temporal change were found
on the surface or in the atmosphere of either Pluto or Charon. Limits
on temporal changes as a function of spatial scale and temporal interval
during the New Horizons encounter are determined. The longest time
interval constraint is one Pluto/Charon rotation period ( ≈ 6.4
Earth days). Contrast reversal and high-phase bright features that
change in appearance with solar phase angle are identified. The change
of appearance of these features is most likely due to the change in
phase angle rather than a temporal change. Had active plumes analogous
to the plumes discovered on Triton been present on the encounter
hemispheres of either Pluto or Charon, they would have been detected.
The absence of active plumes may be due to temporal variability (i.e.,
plumes do occur but none were active on the encounter hemispheres during
the epoch of the New Horizons encounter) or because plumes do not occur.
Several dark streak features that may be deposits from past plumes are
identified.

Charon, the largest moon of Pluto, appeared as a fairly homogeneous, gray, icy world to New Horizons
during closest approach on July 14th, 2015. Charon's sub-Pluto hemisphere was scanned by the Ralph/LEISA
near-IR spectrograph providing an unprecedented opportunity to measure its surface composition.
We apply a statistical clustering tool to identify spectrally distinct terrains and a radiative transfer
approach to study the variations of the 2.0-μm H2O ice band. We map the distribution of the ices
previously reported to be present on Charon's surface, namely H2O and the products of NH3 in H2O.
We find that H2O ice is mostly in the crystalline phase, confirming previous studies. The regions with
the darkest albedos show the strongest signature of amorphous-phase ice, although the crystalline component
is still strong. The brighter albedo regions, often corresponding to crater ejecta blankets, are characterized
by larger H2O grains, possibly an indication of a younger age. We observe two different behaviors for
the two absorption bands representing NH3 in H2O. The 2.21-μm band tends to cluster more in
the northern areas compared to the ≈ 2.01-μm band. Both bands are present in the brighter
crater rays, but not all craters show both bands. The 2.21-μm band is also clearly present on the
smaller moons Hydra and Nix. These results hint that different physical conditions may determine the appearance
or absence of these two different forms of NH3 in H2O ice in the Pluto system. We also investigate the
blue slope affecting the spectrum at wavelengths longer than ∼ 1.8 μm previously reported by several
authors. We find that the slope is common among the objects in the Pluto system, Charon, the smaller moons
Nix and Hydra, and the darkest terrains on Pluto. It also characterizes the analog ice tholin obtained from
irradiation of Pluto-specific materials (a mixture of N2, CH4, and CO ices) in the laboratory. Our
modeling results show that Pluto ice tholins are widespread almost uniformly on Charon suggesting a common
distribution possibly part of the original reservoir of materials that made up Charon. This was irradiated
over the years to yield the gray color characteristic of Charon today. On top of the `primordial' Pluto ice
tholin there is the redder component produced by irradiation of the CH4 provided by Pluto’s atmospheric
contribution as illustrated by Grundy et al. (2016, Nature 539, 65).

We investigate the dynamics of the (47171) Lempo triple system, also
known by 1999 TC36. We derive a full 3D N-body model that takes
into account the orbital and spin evolution of all bodies, which are
assumed triaxial ellipsoids. We show that, for reasonable values of the
shapes and rotational periods, the present best fitted orbital solution
is chaotic and unstable in short time-scales. The formation mechanism of
this system is unknown, but the orbits can be stabilised when tidal
dissipation is taken into account. The dynamics of this system is very
rich, but depends on many parameters that are presently unknown. A
better understanding of this systems thus requires more observations,
which also need to be fitted with a complete model like the one
presented here.

Observations of clustering among the orbits of the most distant
trans-Neptunian objects (TNOs) has inspired interest in the possibility
of an undiscovered ninth planet lurking in the outskirts of the solar
system (Trujillo & Sheppard 2014; Batygin & Brown 2016a). Numerical
simulations by a number of authors have demonstrated that, with
appropriate choices of planet mass and orbit, such a planet can maintain
clustering in the orbital elements of the population of distant TNOs,
similar to the observed sample. However, many aspects of the rich
underlying dynamical processes induced by such a distant eccentric
perturber have not been fully explored. We report the results of our
investigation of the dynamics of coplanar test-particles which interact
with a massive body on an circular orbit (Neptune) and a massive body
on a more distant, highly eccentric orbit (the putative Planet Nine). We
find that a detailed examination of our idealized simulations affords
tremendous insight into the rich test-particle dynamics that are
possible. In particular, we find that chaos and resonance overlap plays
an important role in particles' dynamical evolution. We develop a simple
mapping model that allows us to understand in detail the web of
overlapped mean-motion resonances explored by chaotically evolving
particles. We also demonstrate that gravitational interactions with
Neptune can have profound effect on the orbital evolution of particles.
Our results serve as a starting point for a better understanding of the
dynamical behavior observed in more complicated simulations that can be
used to constrain the mass and orbit of Planet Nine.

Laboratory experiments indicate that direct growth of silicate grains
via mutual collisions can only produce particles up to roughly
millimeters in size. On the other hand, recent simulations of the
streaming instability have shown that mm/cm-sized particles require an
excessively high metallicity for dense filaments to emerge. Using a
numerical algorithm for stiff mutual drag force, we perform simulations
of small particles with significantly higher resolutions and longer
simulation times than in previous investigations. We find that particles
of dimensionless stopping time τs = 10−2 and 10−3
- representing cm- and mm-sized particles interior of the water ice
line - concentrate themselves via the streaming instability at a solid
abundance of a few percent. We thus revise a previously published
critical solid abundance curve for the regime of τs << 1. The solid density in the concentrated regions reaches values higher
than the Roche density, indicating that direct collapse of particles
down to mm sizes into planetesimals is possible. Our results hence
bridge the gap in particle size between direct dust growth limited by
bouncing and the streaming instability.

This scientific workshop highlights the current knowledge and understanding
of the Transneptunian Solar System. We invite you to register for the meeting and to
propose contributed papers for the workshop sessions. The deadline for registration
and abstract submissions is 2018 January 20. Presentations will cover the following topics:

Details on the workshop framework (SOC, LOC, invited speakers,
deadlines, venue, and travel and hotel information) as well as access for
registration, hotel booking and abstract submission can be found at
http://www2.mps.mpg.de/services/coimbra/

Newsletter Information

The Distant EKOs Newsletter is dedicated to provide researchers with
easy and rapid access to current work regarding the Kuiper belt (observational
and theoretical studies), directly related objects (e.g., Pluto, Centaurs), and
other areas of study when explicitly applied to the Kuiper belt.

We accept submissions for the following sections:

Abstracts of papers submitted, in press, or recently published in refereed journals

Titles of conference presentations

Thesis abstracts

Short articles, announcements, or editorials

Status reports of on-going programs

Requests for collaboration or observing coordination

Table of contents/outlines of books

Announcements for conferences

Job advertisements

General news items deemed of interest to the Kuiper belt community

A LaTeX
template for submissions is appended to each issue of the newsletter, and
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template, and send your submission to:

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issues of the Newsletter are archived there in various formats. The web
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Distant EKOs is not a refereed publication, but is a tool for
furthering communication among people interested in Kuiper belt research.
Publication or listing of an article in the Newsletter or the web page does
not constitute an endorsement of the article's results or imply validity of its
contents. When referencing an article, please reference the original source;
Distant EKOs is not a substitute for peer-reviewed journals.

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